This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Metals in general, and in particular rare earth elements (REE's), are critical components of many high-valued products, such as petroleum refining catalysts, phosphors in color television and flat panel displays (cell phones, portable DVDs, and laptops), permanent magnets, and rechargeable batteries for hybrid and electric vehicles. Rare earth elements consists of 15 lanthanides (Ln's), scandium and yttrium. Currently, the REE's used in the U.S. are primarily imported from China, which produces more than 90% of the REE's used globally. Since China has reduced the export quota almost by half since 2010, it is highly desirable to develop efficient and cost-effective processes to produce and recover REE's domestically.
As an example, a typical production process for the rare earth elements can include the following steps: (1) physical separations (gravity concentration, flotation, magnetic, or electrostatic separation) which are used to separate rare earth minerals from sands and rocks in the ore; (2) dissolution of rare earth minerals in acidic or caustic solutions; (3) separation of each REE element from the mixture solutions; (4) precipitation of each REE element using oxalic acid to obtain solid REE oxalate, which is then decomposed under heat to form REE oxide of a single element. Among these steps, Step (3) is most challenging and costly because many of the REE's are present in the solution, and they have very similar chemical properties, ionic sizes, and charges.
The current large-scale production of REE's is mainly based on solvent extraction. Almost 20 sequential or parallel extraction steps using organic solvents (naphthenic acid or phosphorous-based extractants) and strong acids (hydrochoric acid or sulfuric acid) are needed to separate the REE's into eight or ten major fractions. Such a method requires large amounts of organic extractants and highly acidic or caustic aqueous solutions, which generates a lot of environmentally-hazardous wastes.
An alternative method to separate REE's is ligand-assisted displacement chromatography using an ion exchanger. In this method, the REE's are loaded onto a strong-acid cation exchange resin, and then displaced by sodium or ammonium ions in the presence of a ligand. In order to increase the purity and yield, a large column (0.45 L), a large amount of ligand solution (>130 column volumes), and a long displacement time (>3 weeks) are required to separate a small amount of REE's (<2 g), resulting in low productivity and poor ligand efficiency. Worse still, after each run, the column needs to be regenerated by a concentrated solution of acid or transition metal salt, which increases the operation cost significantly. As a result, this method is estimated to have a production cost of 40/kg, which is not economical for large-scale productions.
Another method to achieve REE's separation is extraction chromatography, in which a chelating agent is immobilized onto a resin to increase the selectivity of the sorbent for the REE's. The resins were developed by Argonne National Laboratory in the 1970's, and have been tested in analytical chromatography. Column test data showed that two small columns (with 0.3 g resin) can be used in tandem to capture and purify six REE's using two pH elution steps. However, the resin supply is limited at present, and the resin life is not well evaluated. Most importantly, the resin cost is over 16,000/kg, which is highly uneconomical for large-scale REE's separation.
There is therefore an unmet need for an efficient, cost effective method and system for achieving rare earth metal ion separation.